Referring to FIG. 5, an electron beam B, or a probe, is caused to hit a specimen S having an irregular surface at a incident angle .phi.. The probe B has a diameter of d.sub.p. The spacing between successive raster lines is S.sub.r. In the prior art scanning electron microscope, electron probe microanalyzer, Auger electron spectrometer, or the like, after the probe current I.sub.p illuminating the specimen S has been determined, the electron optics is so controlled that the probe diameter d.sub.p on the surface of the specimen S is reduced, or brought close, to a minimum, as described in Japanese Patent Publication No. 10740/1981 and Japanese Patent Laid-Open No. 159943/1989. As a result, where the magnification M is low, the spacing S.sub.r between the successive raster lines of the electron probe B scanning the surface of the specimen S is much larger than the probe diameter d.sub.p. Also, the angle of convergence .phi. of the beam B impinging on the specimen S assumes a large value, e.g., 0.5 to 1.times.10.sup.-2 rad. Therefore, the depth of focus given by EQU D.sub.F =.+-.(r/M-d.sub.p)/2.phi. (1.1)
takes a small value. In the above equation, r is the value of blur permitted by the human eye. It is known that this value of blur r is usually 0.2 mm. Where d.sub.p &lt;&lt;r/M and M=100 x, we have r/M=2.times.10.sup.-3 mm. By substituting 2.phi.=1 to 2.times.10.sup.-2 rad., the maximum depth of focus is given by D.sub.F =.+-.0.1 to 0.2 mm. Since many specimens investigated by scanning electron microscopy have surface roughnesses exceeding .+-.0.2 mm, the depth of focus is not always satisfactory.
In the prior art scanning electron microscope, the objective lens and other lenses project and focus a reduced image of the electron gun onto the specimen, the image being also called a virtual source of the electron gun. Conceivable methods of increasing the depth of focus in this lens system projecting the reduced image of the gun are as follows:
(A) Reducing the diameter of the objective aperture 5 (FIG. 2) located before the objective lens, for example, down to about 10 .mu.m.
(B) Increasing the working distance, for example, to about 100 mm.
(C) The present mode of operation in which the probe diameter d.sub.p is minimized for any desired value of the probe current I.sub.p is switched to a second mode of operation in which the depth of focus is always maximized for any desired value of the probe current I.sub.p and any desired value of the magnification M.
The above-described method (C) which is based on a new concept is described, for example, in Japanese Patent Laid-Open No. 236563/1989. This method (C) is next described in detail by referring to FIG. 6 which is similar to FIG. 5. The probe diameter d.sub.p is smaller than the spacing S.sub.r between the raster lines on the surface of the specimen S as long as the magnification M is small even if the probe diameter d.sub.p is increased. The method (C) makes use of this principle. Usually, the spacing S.sub.r is so selected that it cannot be perceived by the human eye even if it is magnified at the magnification M (S.sub.r .multidot.M.ltoreq.0.2 mm) and, therefore, the relation d.sub.p .multidot.M&lt;0.2 mm holds. Thus, the blurring caused by the increased probe diameter d.sub.p is not perceived. In this way, the depth of focus is made greatest within the range in which the blur is not perceived.
In an ultra-high resolution scanning electron microscope using a strongly excited objective lens to provide higher magnification than the conventional scanning electron microscope, the gap between the polepieces of the objective lens is made small to reduce the aberration coefficient of the lens. The specimen is placed within this gap. In this case, it is necessary to weaken the excitation of the objective lens to decrease the magnification. For this purpose, any one of the following procedures has been adopted.
(D) The objective lens is deenergized. The focus is adjusted by the condenser lens located at the previous stage.
(E) The excitation of the objective lens is changed according to the magnification. As the magnification is decreased, the excitation of the objective lens is weakened. The focus is adjusted by changing the focal length of the condenser lens positioned ahead of the objective lens. At this time, the beam diameter on the principal plane of the objective lens is limited by the diameter of the aperture of the condenser lens. This method is disclosed, for example, in Japanese Patent Publication No. 40380/1974.
In the method (D) described above, the distance between the condenser lens at the previous stage and the specimen is large, e.g., 150 mm. Therefore, the angle of convergence .phi. of the beam impinging on the specimen can be readily set to 1.times.10.sup.-3 rad. by inserting an aperture 300 .mu.m in diameter into the principal plane of the condenser lens at the previous stage. In the method (E), it can be seen that the depth of focus can be made large by controlling the condenser lens at the previous stage in such a way that the beam diameter on the principal plane of the objective lens is reduced, as described in Japanese Patent Publication No. 36769/1974. The methods (D) and (E) are primarily intended to permit observation at low magnifications. The above-described increase in the depth of focus is by the secondary effect.
The method (A) described above needs several operations. First, the aperture must be replaced. Then, it is necessary to adjust the axis of the aperture. Where it is desired to increase the depth of focus fivefold, for example, the aperture diameter must be reduced at least by a factor of five. This presents problems concerning manufacturing accuracy. Also, if the aperture is contaminated, the beam passing through the aperture is adversely affected. In an instrument where the focal point of the second condenser lens 2 (FIG. 2) is brought close to the aperture 5 to increase the probe current, the angle of convergence .phi. of the beam incident on the specimen increases with increasing the probe current.
Where the method (B) is effected, an operation for varying the working distance is necessitated. In addition, the field of view shifts, because the electric field produced by the secondary electron detector affects the electron probe in a longer space. This effect becomes more conspicuous when the accelerating voltage is reduced.
In the method (C) described above, an ideal control is provided. However, if the focal point of the condenser lens located ahead of the objective lens is situated between the objective lens and the condenser lens at the previous stage and shifts, then the focal length of the objective lens may vary greatly. In this case, it is necessary to correct the rotation of the image in step with the excitation of the objective lens.
In the method (D) described above, the excitation of the objective lens changes from zero to the greatest extent. Therefore, misalignment of the optics takes place. Sometimes, the field of view shifts more than about 50 .mu.m. Furthermore, the efficiency at which secondary electrons are detected deteriorates when the excitation of the objective lens is reduced down to zero, since the secondary electron detector is positioned above the upper polepiece of the objective lens. As a result, the image darkens. Where the focus is adjusted by means of the condenser lens located ahead of the objective lens, if the objective aperture does not exist at the principal plane of this condenser lens, the probe current is varied by the adjustment. This changes the brightness of the image. Consequently, it may be difficult to make the adjustment.
Where the method (E) described above is implemented, if the working distance changes greatly, i.e., if the distance w between the principal plane of the objective lens 3 and the specimen surface is in excess of the focal length f of the objective lens 3 as shown in FIG. 7, then the distance a (see FIG. 1) from the principal plane of the objective lens 3 to the focal point Q of the second condenser lens 2 located ahead of the objective lens 3 as measured in the direction of travel of the beam is given by EQU a=f w/(f-w)&lt;0 (1.2)
That is, the beam spread at the principal plane becomes larger. Under the condition to maintain a large depth of focus, it is impossible to keep up with the focus only by a continuous adjustment of the condenser lens 2. In the case of strongly excited objective lens, a change in the excitation of the objective lens leads to a modification of the lowest magnification attainable. Moreover, a large change in the working distance directly results in a change in the focal length of the objective lens, because the distance between the object lens principal plane and the specimen plane is extremely short. This complicates the movement made to correct the rotation of the image.
The present applicant has already proposed a depth-of-focus-adjusting apparatus for use in an electron microscope or the like in Japanese Patent Application Serial No. 248794/1989. In particular, an electron gun, a first condenser lens, a second condenser lens, an objective aperture, and an objective lens are arranged in this order in the direction of travel of the electron beam. A large depth of focus is obtained by controlling the focal length of the first and second condenser lenses without modifying the relation of the focal length of the objective lens to the probe current. In this proposed apparatus, the mode of operation can be switched from this new mode of operation to the conventional high-resolution mode without changing the working distance or the diameter of the objective aperture to permit various observations.
In this apparatus proposed in Japanese Patent Application Serial No. 248794/1989, when the mode of operation is switched from the conventional high-resolution mode to the large depth-of-focus mode, the image rotates. Also, the image rotates if the position of the specimen relative to the objective lens differs, and if the mode of operation is switched similarly from the conventional mode to the large depth-of-focus mode. However, these rotations of image are not taken into consideration.